1. Field of the Invention
The present invention relates in general to a microwave oven, and more particularly to a device for sensing and measuring, by the capacitance or the electrostatic capacity, a weight of food laid on a turntable in a cooking chamber of the microwave oven.
2. Description of the Prior Art
There have been proposed several types of devices for sensing a weight of food laid on a turntable in a cooking chamber of a microwave oven. A description for a representative example of the food weight sensing devices of the microwave ovens will be given referring to the accompanying drawing, FIG. 10.
FIG. 10 shows in a sectional view a food weight sensing device for a microwave oven according to the prior art. As shown in this drawing, the known food weight sensing device comprises a base plate 31 of a cooking chamber, the cooking chamber receiving food to be cooked, and a lower support plate 32. The weight sensing device further comprises a rubber plate 33 arid a pair of strong dielectric plates 34 and 35 orderly interposed between the base plate 31 and the lower support plate 32. Here, the rubber plate 33 is provided with upper and lower hemispherical protrusions 33a and 33b of which the upper protrusion 33a is inserted in a through hole 31a of the base plate 31.
In the drawing, the reference numeral 36 denotes a roller of a rotary ring supporting the turntable (not shown) on which the food is laid to be cooked by microwaves emitted from a magnetron (not shown) of the microwave oven.
In operation of the above food weight sensing device, when the roller 36 of the rotary ring supporting the turntable passes on the upper protrusion 33a of the rubber plate 33 inserted in the hole 31a of the cooking chamber base plate 31, the food weight is acted on rubber plate 33 and this causes the lower protrusion 33b of the rubber plate 33 to press down the pair of strong dielectric plates 34 and 35. A piezoelectric effect is thus caused by the strong dielectric plates 34 and 35, pressed down by the lower protrusion 33b of the rubber plate 33, and a capacitance is sensed by the piezoelectric effect. Hence, the weight of the food laid on the turntable is sensed and measured.
However, the above food weight sensing device for the microwave oven has a problem caused by the pair of strong dielectric plates 34 and 35. That is, the pair of strong dielectric plates 34 and 35 have different capacitances and this causes a complex peripheral circuit to be necessarily added to the construction of the microwave oven to sense the desired capacitance, nevertheless, a precise weight sensing result can not be available. Furthermore, each of the strong dielectric plates 34 and 35 has a bad heat resistance as well as a bad shock resistance due to its intrinsic characteristics.